About histograms

A histogram illustrates
how pixels in an image are distributed by graphing the number of
pixels at each color intensity level. The histogram shows detail
in the shadows (shown in the left part of the histogram), midtones
(shown in the middle), and highlights (shown in the right part)
A histogram can help you determine whether an image has enough detail
to make a good correction.

The histogram also gives a quick picture of the tonal range of
the image, or the image key type. A low‑key image has
detail concentrated in the shadows. A high‑key image has detail
concentrated in the highlights. And, an average-key image has detail
concentrated in the midtones. An image with full tonal range has
some pixels in all areas. Identifying the tonal range helps determine
appropriate tonal corrections.

The Histogram panel offers many options for viewing tonal and color information about an image. By default, the histogram displays the tonal range of the entire image. To display histogram data for a portion of the image, first select that portion.

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You can view an image histogram as an overlay in the Curves dialog box by selecting the histogram option under Curve Display Options, and in the Curves Properties panel, or by choosing Curve Display Options from the panel menu, then Histogram.

Histogram panel overview

Choose
Window > Histogram or click the Histogram tab to open
the Histogram panel. By default, the Histogram panel opens in Compact
View with no controls or statistics, but you can adjust the view.

Adjust the view of the Histogram
panel

Choose a view from
the Histogram panel menu.

Expanded View

Displays the histogram with statistics. It also displays:
controls for choosing the channel represented by the histogram,
viewing options in the Histogram panel, refreshing the histogram
to display uncached data, and choosing a specific layer in a multilayered
document.

Compact View

Displays a histogram with no controls or statistics.
The histogram represents the entire image.

All Channels View

Displays individual histograms of the channels in addition to all the options of the Expanded View. The individual histograms do not include alpha channels, spot channels, or masks.

Histogram panel with all channels displayed in color and statistics hidden

View a specific channel in the
histogram

If you chose the Expanded View
or All Channels View of the Histogram panel, you can choose a setting
from the Channel menu. Photoshop remembers the channel setting if
you switch from either Expanded View or All Channels View back to
Compact View.

Choose an individual channel to display
a histogram of the channel, including color channels, alpha channels,
and spot channels.

Depending on the color mode of the image, choose RGB,
CMYK, or Composite to view a composite histogram of all the channels.

If the image is RGB or CMYK, choose Luminosity to display
a histogram representing the luminance or intensity values of the
composite channel.

If the image is RGB or CMYK, choose Colors to display
a composite histogram of the individual color channels in color.
This option is the default view for RGB and CMYK images when you
first choose Expanded View or All Channels View.

In the All Channels View, choosing from the Channels menu
affects only the topmost histogram in the panel.

View channel histograms in color

From the Histogram panel, do one
of the following:

In the All Channels View, choose Show Channels
In Color from the panel menu.

In Expanded View or All Channels View, choose an
individual channel from the Channel menu and choose Show Channels
In Color from the panel menu. If you switch to Compact View, the
channel continues to be shown in color.

In Expanded View or All Channels View, choose Colors
from the Channel menu to show a composite histogram of the channels
in color. If you switch to Compact View, the composite histogram
continues to be shown in color.

View histogram statistics

By
default, the Histogram panel displays statistics in the Expanded
View and All Channels View.

Choose Show Statistics from the Histogram panel
menu.

Do one of the following:

To view information about a specific pixel
value, place the pointer in the histogram.

To view information about a range of values, drag
in the histogram to highlight the range.

The panel displays the following statistical information
below the histogram:

Mean

Represents
the average intensity value.

Std Dev (Standard deviation)

Represents how widely
intensity values vary.

Median

Shows the middle value in the range
of intensity values.

Pixels

Represents the total number of pixels used to calculate the histogram.

Level

Displays the intensity level of the area underneath the
pointer.

Count

Shows the total number of pixels corresponding to the
intensity level underneath the pointer.

Percentile

Displays the cumulative number of pixels at or below
the level underneath the pointer. This value is expressed as a percentage
of all the pixels in the image, from 0% at the far left to 100%
at the far right.

Cache Level

Shows
the current image cache used to create the histogram. When the cache
level is higher than 1, the histogram is displayed faster. In this case,
the histogram is derived from a representative sampling of pixels
in the image (based on the magnification). The original image is
cache level 1. At each level above level 1, four adjacent pixels
are averaged to arrive at a single pixel value. So, each level is
half the dimensions (has 1/4 the number of pixels) of the lower
level. When Photoshop makes a quick approximation, it can use one
of the upper levels. Click the Uncached Refresh button to redraw
the histogram using the actual image layer.

View the histogram for a multilayered
document

Choose Expanded View from the Histogram
panel menu.

Choose a setting from the Source menu. (The Source menu
is not available for single-layered documents.)

Entire Image

Displays a histogram of the entire image, including all
layers.

Selected Layer

Displays a histogram of the layer that’s selected in
the Layers panel.

Adjustment Composite

Displays a histogram of an adjustment layer selected
in the Layers panel, including all the layers below the adjustment
layer.

Preview histogram adjustments

You can preview the effect on the histogram
of any color and tonal adjustments.

Select the Preview option in the dialog boxes
of any color or tonal adjustment command.

When Preview is selected, the Histogram panel shows how
the adjustment affects the histogram.

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When making adjustments
using the Adjustments panel, changes are automatically reflected
in the Histogram panel.

Refresh the histogram display

When a histogram is read from a cache instead of the current state of the document, the Cached Data Warning icon appears in the Histogram panel. Histograms based on the image cache are displayed faster and are based on a representative sampling of pixels in the image. You can set the maximum cache level (from 2 to 8) in the Performance preference.

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A higher cache level setting will increase the redraw speed for large, multi-layer files, but requires additional usage of system RAM. If RAM is limited or you work mainly with smaller images, use lower cache level settings

To refresh the histogram so that it displays all
of the pixels of the original image in its current state, do one
of the following:

View color values in an image

You can use the Info panel to see the color value of pixels as you make color corrections. When you work with the Properties panel, the Info panel displays two sets of color values for the pixels under the pointer. The value in the left column is the original color value. The value in the right column is the color value after the adjustment is made.

Using Levels and Info panel to neutralize the tone of an image

You can view the color of a single
location using the Eyedropper tool . You
can also use up to four Color Samplers to
display color information for one or more locations in the image.
These samplers are saved in the image, so you can refer to them
repeatedly as you work, even if you close and reopen the image.

Color samplers and Info panel

Choose
Window > Info to open the Info panel.

Select (then Shift-click) the Eyedropper tool or
Color Sampler tool , and
if necessary, choose a sample size in the options bar. Point Sample
reads the value of a single pixel, other options read the average
of a pixel area.

If you selected the Color Sampler tool , place
up to four color samplers on the image. Click where you want to
place a sampler.

View color information while adjusting
color

You can view color information for specific pixels in the image while adjusting color in the Properties panel.

Add an adjustment using the Adjustments panel.

Make adjustments in the Properties panel. As you make adjustments, view the before and after color values in the Info panel. Move the pointer over the image to view color values at the pointer location.

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If
you are using a command from the Image > Adjustments menu, the Eyedropper
tool is
activated (and other tools temporarily disabled) when you move the
pointer over the image. You still have access to the scroll controls
and to the Hand and Zoom tools
using keyboard shortcuts.

If you’ve placed color samplers on the image, the color
values under the color samplers appear in the lower half of the
Info panel. To add new color samplers, select the Color Sampler
tool and click in the image, or select the Eyedropper tool and Shift-click
in the image.

Adjusting color samplers

Once you’ve added a color sampler, you can
move or delete it, hide it, or change the color sampler information
displayed in the Info panel.

Move or delete a color sampler

Select
the Color Sampler tool .

Do one of the following:

To move a color sampler, drag the sampler
to the new location.

To delete a color sampler, drag the sampler out
of the document window. Alternatively, hold down Alt (Windows) or
Option (Mac OS) until the pointer becomes a scissors and
click the sampler.

To delete all color samplers, click Clear in the
options bar.

To delete a color sampler while an adjustment dialog
box is open, hold down Alt+Shift (Windows) or Option+Shift (Mac OS),
and click the sampler.

Hide or show color samplers in
an image

Change the display of color sampler
information in the Info panel

To display or hide color sampler
information in the Info panel, choose Color Samplers from the panel
menu. A check mark indicates that the color sampler information
is visible.

To change the color space in which a color sampler displays
values, move the pointer onto the color sampler icon in
the Info panel. Then, hold down the mouse button, and choose another
color space from the menu.